1. Field of the Invention
The present invention relates to a printing apparatus and a printing data producing method, and specifically, to a printing apparatus performing printing in which gradation levels of a print image are expressed by a combination of different sizes of printed dots and production of printing data used in such printing apparatus.
2. Description of the Related Art
As a representative of such a printing apparatus, an ink jet printing apparatus that prints an image by applying inks of the same color and of a plurality of different ejection amounts is known. Printing data used in this ink jet printing apparatus is obtained through a conversion of image data which expresses gradations in a multi-level form (for example, 0-255 by 8 bits) into final ejection data in a binary level form, for each pixel. For example, the image data is, based on a value shown thereby, converted into pattern data of plural bits which expresses the gradation of one of several levels and further index patterns, which express predetermined dot arrangements for respective levels expressed by the pattern data, are used to obtain the binary ejection data for forming dots of the arrangement. Thus, the gradation and maximum density of a printed image can be set by determining the index patterns appropriately.
For example, in a configuration that the image data of 256 gradation levels 0-255 is converted into the pattern data of 4 bits (expressed as 0000-0101 of 4 bits) which expresses one of 9 values (levels 0-8), and the converted pattern data is converted into the binary data by using the index pattern corresponding to the converted pattern data, the index patterns are set so that large and small dots are arranged correspondingly to the respective 9 levels. Thus, the multi-value image data can be converted into the ejection data (binary data) for each nozzle of a printing head, which corresponds to a large or small size of an ink droplet ejected from the printing head.
The index pattern is one of the factors that determine characteristics of a printed image, such as a gradation image. In general, since a highlight portion of the printed image appears to be more granular when a relatively large amount of ink is used in this portion, the index patterns are such that, for example, smaller dots are arranged up to one of the nine levels which corresponds to an intermediate gradation range and larger dots start to be arranged at the next higher level corresponding to the next larger gradation value.
However, the index patterns are such that the arrangements of larger and smaller dots are uniformly determined therebetween correspondingly to pattern data and the larger and smaller dots are uniformly assigned to each level over the range of gradation values that can be expressed by image data. Accordingly, it is likely that only smaller dots are arranged at levels with smaller gradation values.
As a result, in a highlight portion of the image or an intermediate gradation portion with a higher density than that of the highlight portion, which portions are expressed by levels with only such smaller dots arranged thereat, the following problems may occur. Since a relatively small amount of ink (small droplet) forming small dots has relatively low kinetic energy induced by ejection, vibration of a mechanical portion associated with a printing operation or an air stream occurring when the printing head moves may cause an ejecting state of the small droplet to be disturbed (that is, biased), thereby causing deviation of positions of dots formed. This deviation is recognized as a decrease in quality of the printed image. In particular, since the intermediate gradation portion has a higher dot density than the highlight portion, in the former portion, stripes (bands) or the like due to the deviation of the formed dot become more noticeable.